This invention relates generally to electrolytes for electrochemical cells and storage batteries, and more particularly to polymer gel electrolytes for such cells.
There has been a great deal of interest in developing better and more efficient methods for storing energy for applications such as radio communications, satellites, portable computers, and electric vehicles, to name but a few. Accordingly, recently there have been concerted efforts with the aim of developing high energy, low weight cost-effective batteries having improved performance characteristics.
Rechargeable, or secondary cells are more desirable than primary (non-rechargeable) cells for use in certain applications since the associated chemical reactions which take place at the positive and negative electrodes of the battery are reversible. Electrodes for secondary cells are capable of being regenerated (i.e. recharged) many times by the application of an electrical charging current thereto. Numerous advanced electrode systems have been developed for storing electrical charge in chemical form. Concurrently, much effort has been dedicated to the development of electrolytes capable of enhancing the capabilities of electrochemical cells.
Heretofore, electrolytes have been either liquid electrolytes as are found in conventional wet cell batteries, such as lead-acid or nickel-cadmium cells, or solid films as are available in newer, more advanced battery systems. Each of these systems has inherent limitations and related deficiencies which make them unsuitable for particular applications.
Liquid electrolytes, while demonstrating acceptable ionic conductivity, tend to leak out of the cells into which they are sealed. While better manufacturing techniques have lessened the occurrence of leakage, cells still do leak potentially dangerous liquid electrolytes from time-to-time. This is particularly true of the currently available lithium-based cells. Moreover, leakage from the cell lessens the amount of available electrolyte in the cell, thus reducing the usefulness of the cell. Cells using liquid electrolytes are also not available for all sizes and shapes of batteries.
By contrast, solid electrolytes are substantially free from problems of leakage. However, they have generally have much lower conductivities as compared to liquid electrolytes. For example, conventional solid electrolytes have ionic conductivities in the about 10xe2x88x925 S/cm (Siemens per centimeter), whereas for many applications an ionic conductivity  greater than 10xe2x88x923 S/cm is required. Good ionic conductivity is necessary to ensure a battery system capable of delivering requisite amounts of power for a given application. For example, good conductivity is necessary for the high rate operation demanded by cellular telephones and satellites. Accordingly, solid electrolytes are inadequate for use in many high-performance battery systems.
While solid electrolytes are intended to replace the combination of liquid electrolytes and separators used in conventional batteries, the above-described limitations have prevented them from being fully implemented. One class of solid electrolytes, specifically gel electrolytes, have shown some promise. Gel electrolytes contain a significant fraction of solvents (or plasticizers) in addition to a salt and a polymer. In recent years, there has been an increasing trend to replace conventional Nixe2x80x94Cd batteries with lithium batteries, particularly those utilizing gel-polymer electrolytes. Advantages of such batteries include, for example, a high voltage (3-4V), a high power density; a low self-discharge (less than 1% per year), a long operation time (because gel-polymer electrolytes do not leak or decompose upon use), a high performance efficiency (85-95%), and a wide range of operating temperatures (from about xe2x88x9250xc2x0 C. to about 50xc2x0 C.).
These advantages make lithium batteries promising power sources for battery-powered automobiles. While Nixe2x80x94Cd batteries provide only a 60-150 km operating range and account for about 30% of the car""s weight, lithium batteries can provide a 450 km range and are much lighter. Unfortunately, standard liquid electrolytes used in conventional lithium batteries are aggressive with respect to the cathode and anode. Upon cycling, an oxide film forms on the lithium anode, which eventually renders the battery inoperative. Another problem is the growth of dendrites on the lithium anode, which may short-circuit it to the cathode.
Despite the advantages of gel-polymer electrolytes, the lithium electrode still undergoes passivation which decreases the life expectancy of the battery. This problem has presented a significant limitation to the successful implementation of gel polymer electrolytes in electrochemical cells and storage batteries.
Accordingly, there exists a need for a new polymer gel electrolyte system which combines the properties of good mechanical integrity, as well as the ability to absorb sufficient amounts of liquid electrolytes so as to have a high ionic conductivity comparable to that of liquid electrolytes. The desired electrolytes should also avoid or limit electrode passivation, as well as each of the above described problems associated therewith, such as decreased life expectancy of the battery.